The present invention relates to drivers for fluxgate magnetometers and more particularly to a low power, low noise, unidirectional pulse driver.
Fluxgate magnetometers require that electrical current be supplied periodically to a winding to magnetically saturate the magnetometer core. The presence of an applied magnetic field, such as the earth's field or the field of a magnetic body, is detected by sense windings on the magnetometer core as the core cycles in and out of saturation producing a time-varying permeability. This process consumes some energy E per cycle. Magnetic material volume and other material parameters as well as core geometry and drive circuit efficiency control the magnitude of this energy. Expenditure of energy E permits one sample of the ambient magnetic field amplitude to be registered in an output circuit and given a numerical value. This numerical value varies from cycle to cycle because of fluctuations in the magnetic material parameters and in the circuitry supplying the saturation drive current. If N samples per second are taken and averaged, a more stable value for the field measurement is obtained in some desired bandwidth. For a specified bandwidth and field measurement instability (noise), a power N.multidot.E must be continuously expended to operate the magnetometer. If the noise is uncorrelated from cycle to cycle, a specific core and driving circuit yield, by the averaging process, a magnetometer signal-to-noise ratio proportional to .sqroot.N.
Prior art energy efficient magnetometer drivers include devices such as the oscillator described in U.S. Pat. No. 3,649,908. The oscillator described therein operates in the free-running flyback mode, i.e., the ON interval of the transistor switch is substantially longer than the OFF interval. The repetition rate or frequency of the oscillator is largely determined by the supply voltage V and the inductance L.sub.1 of the magnetometer drive winding. In typical implementations, this frequency maybe 20 KHz to 50 KHz and the power consumption about 5 milliwatts. This oscillator is a stable, efficient, and low-noise source of the current pulses necessary for polarizing the magnetic core in fluxgate magnetometers. Its major drawback is the high repetition rate which fixes the sampling rate of the magnetometer and also its power consumption. For detection of slowly changing magnetic signals, a sampling rate of a few hundred Hertz would be adequate. The sampling rate required to yield an adequate signal-to-noise ratio depends on the application. When operating in the free-running flyback mode, the oscillator produces a magnetometer with about 1/4 gamma noise. If the repetition rate could be reduced by a factor of ten without changing the desirable features of the oscillator, power consumption would be reduced to one-tenth and noise increased by perhaps the .sqroot.10 to slightly less than one gamma.
Copending application Ser. No. 157,925 now U.S. Pat. No. 4,384,254, entitled Micro-Power Magnetometer, filed by the present inventor on June 6, 1980, discloses an attempt at decreasing the oscillator repetition rate (to thereby reduce the product N.multidot.E) by using the closely related blocking oscillator as a driver. This implementation produced a useful magnetometer with very low power consumption, near 100 microwatts. The low-noise features of the flyback mode of oscillation were sacrificed, however, and noise was much higher than predicted from the square root of the ratio of repetition rates.
As set forth in application Ser. No. 157,925, features of a driving oscillator deemed necessary for fluxgate magnetometer operation at very low power expenditure are (1) a repetition rate controllable independent of core winding inductances and (2) a driving current-on interval terminated only by core saturation.
The magnetometer driver of the present invention, as will more fully appear below, retains the low power features of the blocking oscillator driver while at the same time retaining the low noise feature of the flyback mode of operation.